March 16, 2012

p - Block Elements - II Three Marks


1. Draw / Give the electronic structure of following i) PCl5 ii) H3PO3
2. How is phosphoric acid prepared in the laboratory?
Phosphoric acid (H3PO4) is prepared by boiling a mixture of red phosphorus with 50% Nitric acid in the presence of iodine as catalyst in a flask fitted with a reflux condenser on a water bath till no more oxides of nitrogen are liberated.
                            I2
P +5HNO3  H3PO4 +5NO2 + H2O
3. How is potash alum prepared?
Alunite or alum stone (K2SO4. Al2(SO4)3. 4Al(OH)3) is finely powdered and boiled with dilute Sulphuric acid.
The aluminium hydroxide part changes into aluminium sulphate.
When a little more potassium sulphate in calculated amount is added, the alum is crystallised.
4. How is Xenon fluorides prepared?
Xenon forms three binary Fluorides XeF2, XeF4, and XeF6 by the direct union of elements under appropriate experimental conditions.
           673K
Xe+F2  XeF2
              673K
Xe + 2F2   XeF4
              573K
Xe+ 3F2   XeF6
5. Prove that P2O5 is a powerful dehydrating agent. Or Illustrate the dehydrating property of phosphorous pentoxide (P2O5) with two examples. Or P2O5 is a dehydrating agent. Why?
Phosphorus pentoxide (P2O5) extracts water from many inorganic compound including sulphuric acid, nitric acid and several organic compounds.
           P4O10
H2SO4 SO3 + H2O

           P4O10
2HNO3 N2O5 + H2O

              P4O10
RCONH2 RCN + H2O
6. Prove that phosphorous acid is a powerful reducing agent.
Because phosphorous acid (H3PO3) has P-H bond it reduces silver nitrate into silver.
         2AgNO3 +  H3PO3 + H2O 2Ag + H3PO4 +2HNO3
7. What are interhalogen compounds? Give the preparation.
Each halogen combines with another halogen to form several compounds known as interhalogen compounds.
Interhalogen compounds are prepared by
1. Direct combination or
2. By the action of a halogen on a lower interhalogen compound.
The product formed depends on the conditions.
                                                  473K
Cl2+ F2 (Equal volume) 2ClF (AX type)
                                          573K
Cl2 + 3F2 (Excess) 2ClF3 (AX3 type)
Br2 + 5F2 (Excess) 2BrF5 (AX5 Type)
                             573K
IF5 + F2 (Excess) IF7 (AX7 Type)
8. What is plumbo solvency? Or Write a note on plumbo solvency. Or Write about Plumbo solvency.
Lead is not attacked by pure water in the absence of air, but water containing dissolved air has a solvent action on it due to the formation of lead hydroxide. This phenomenon is called Plumbo solvency.
            2Pb + O2 + 2H2O 2Pb(OH)2
9. Write about the Holme's signal.
1. Containers having a perforated bottom and a hole at the top filled with Calcium phosphide and Calcium carbide are thrown into the sea.
2. Water enters the container through the bottom and reacts with Calcium carbide and Calcium phosphide to give acetylene and phosphine.
3. Phosphine gets ignited spontaneously as it comes in contact with air and also ignites acetylene. Thus a bright red flame is produced which is accompanied by huge smoke due to the burning of phosphine. This serves as a signal to the approaching ships.
Ca3P2 + 6H2O 2PH3 + 3Ca(OH)2
CaC2   + 2H2O C2H2 + Ca(OH)2
10. Discuss / Illustrate the oxidising power of fluorine.
1. Fluorine is the strongest oxidising agent due to high electron affinity.
2. It oxidises other halide ions to halogens in solution or when dry.
F2 + 2X   2F  + X2 (X  = Cl , Br , I)
            3. The oxidizing power decreases from fluorine to iodine.
            4. Halogen of low atomic number oxidises the halide ion of higher atomic number.
11. Give / Write the uses of Helium.
1. Due to lightness and non-inflammability Helium is used to filling balloons for meteorological observations.
2. Due to lightness it is used in inflating aeroplane tyres.
3. Helium oxygen mixture is used by deep-sea divers in preference to nitrogen oxygen mixtures.
4. A mixture of oxygen and helium is used in the treatment of asthma.
5. Liquid helium is used as cryogenic agent for carrying out various experiments at low temperatures.
6. It is used to produce and sustain powerful super conducting magnets which form essential part of modern NMR Spectrometers and Magnetic Resonance Imaging system (MRI) for clinical diagnosis.
12. Give / Write the uses of neon.
1. Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes.
2. Mixed with helium, it is used to protect electrical instruments from high voltages.
3. It is also used in beacon lights for safety of air navigation as the light possesses fog and storm-penetrating power.
4. Neon light is used in botanical gardens as it stimulates growth and helps the formation of chlorophyll.
13. H3PO3 is diprotic / dibasic. Why?
H3PO3 reacts with alkalis like NaOH to give two types of salts.

H3PO3 +  NaOH NaH2PO3 + H2O
                                                 Sodium dihydrogen phosphite

H3PO3 + 2NaOH Na2HPO3 + 2H2O
                                            Disodium hydrogen phosphite
14. H3PO4 is triprotic / tribasic. Prove.
H3PO4 reacts with alkalis like NaOH to give three types of salts.
H3PO4 +  NaOH NaH2PO4 + H2O
                      Sodium dihydrogen phosphate

H3PO4 + 2NaOH Na2HPO4 + 2H2O
                         Disodium hydrogen phosphate

H3PO4 + 3NaOH Na3PO4 + 3H2O
                                           Sodium phosphate (TSP)
15. What happens when phosphorus acid is heated?
When phosphorus acid is heated, it undergoes auto-oxidation and reduction to form phosphoric acid and phosphine.
                 Δ
4H3PO3  3H3PO4 + PH3
16. What is inert pair effect?
1. The tendency of being less availability for ns electron in bonding.
2. The inert pair effect increases down the group with the increase in atomic number.
17. What is the action of heat on orthophosphoric acid?
On heating orthophosphoric acid gives pyrophosphoric acid at 523 K and at 589 K gives metaphosphoric acid.
                     523K                   589K
2H3PO4  H4P2O7 2HPO3 + H2O
                    H2O

18. How silver nitrate reacts with orthophosphoric acid?

On reaction with silver nitrate, orthophosphoric gives yellow precipitate of silver phosphate.
H3PO4 + 3AgNO3 Ag3PO4 + 3HNO3
19. Why is HF not stored in silica or glass bottles? Write the equation.
Because HF attacks silicates and silica (glass).
Na2SiO3 + 6HF Na2SiF6 + 3H2O
SiO2 + 4HF SiF4 + 2H2O
 20. Write the uses of Fluorine
1. Fluorine is used in the manufacture of a series of compounds known as Freons. These non-toxic, non-combustible and volatile liquids are used as refrigerants in refrigerators, deep freezers and air conditioners.
2. CaF2 is used as flux in metallurgy.
3. NaF is used as a preservative to prevent fermentation and also for preventing dental cavities.
4. SF6 is used as an insulating material in high voltage equipment.
5. Teflon is used as container to store hydrofluoric acid.
6. UF6 is used in the separation of U235 from U238.
21. What is Burnt alum?
When heated, alum melts at 365K and on further heating loses the whole of its water of crystallisation and swells up. The swollen mass so produced is called burnt alum
22. Write the uses of Lead
Lead is used
1. For making lead pipes,
2. For making telegraph and telephone wires,
3. In making bullets and lead accumulators,
4. In lead chambers, for the manufacture of sulphuric acid,
5. For making alloys like solder, pewter and type metal,
6. For preparing Tetraethyl lead (Pb(C2H5)4) which is used as an additive to petrol to prevent knocking.
23. Show that Phosphine is a very good reducing agent
When Phosphine is passed through the salt solutions, corresponding metal is formed.
PH3 + 6AgNO3 + 3H2O
6Αg + 6HNO3 + H3PO
24. Mention uses of Potash alum
1. It is used in purification of water, water proofing of textiles and in dyeing and paper industry.
2. It is also employed to arrest bleeding
25. Write the preparation of ClF, ClF3 and IF7
                                       473K
Cl2+ F2 (Equal volume) 
2ClF
                               573K
Cl2 + 3F2 (Excess)
2 ClF3
                           573K
ΙF5 + F2 (Excess)
ΙF7
26. Draw the electron dot formula of H4P2O7
 

ONE MARKS  FIVE MARKS

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